The invention relates to a picture display device having a display tube whose rear part consists of a cylindrical neck accommodating a device for generating electron beams and whose front part is funnel-shaped with the widest part being present on the front side, and a picture display phosphor screen. The display device also being provided with an electromagnetic deflection unit mounted around the display tube for deflecting electron beams across the display screen and comprising a line deflection coil and a field deflection coil which, when energized, generate magnetic fields having at least a dipole component.
Recently more stringent standards have been introduced for certain types of picture display devices, notably for monitors, with respect to the magnetic interference field which they may produce around them. So far protective shields have sometimes been used in picture display devices such as, for example a metal cone envelope for the combination of display tube and deflection unit, but such protective shields are intended to inhibit the influence of external fields on the display device rather than reducing magnetic interference fields generated by the picture display device. An important source of magnetic interference fields is the line deflection coil because it is operated at radiofrequency currents (frequencies in the range of 10 to 100 kHz) as contrasted to the field deflection coil. It is impossible to design a satisfactorily operating deflection coil that produces no stray field. If the stray field were to be eliminated by means of a protective shield, such a shield would only be effective if the combination of display tube and deflection unit were also shielded on the display screen side.